The present invention relates principally to small computing systems and compatible peripherals, specifically to hand portable or briefcase portable systems. Due to the ongoing trend to increasing computer power and decreasing computer size and cost, a new category of computers has entered the market, that of the hand portable or briefcase portable computer. This new class of data processing apparatus performs all of the functions normally associated with data processing apparatuses such as input, computation and output, and includes the capability of interact with peripheral devices. These peripheral devices may include mass memory devices, additional input devices and additional output devices.
This new category of data processing apparatus presents unique problems of data communications. Because the data processing apparatuses themselves are small and light in weight and will ordinarily be mated with peripherals which are similarly small in size and light in weight, these systems require a communications bus structure which provides an adequate rate of data exchange with a minimum of required data and/or control lines. In general, there is a trade off between the number of individual lines included within such a communications bus and the speed at which data may be communicated. The greater the number of communication lines, the greater the data rate capacity of the communications system. In addition to data lines for exchange of data, the communications bus must include additional lines for transmission of control signals indicating the beginning of a message and the indication of when data upon the data lines is valid. Typically such a system requires one or more lines of the communication bus dedicated for indicating the pendency of communication and a further plurality of communication lines for indicating when transmitted data is valid and indicating the proper receipt of transmitted data.
Because of the known relationship between the number of data lines and the speed of data transmission, the engineering choice between complexity and speed of transmission is well known. However, if the number of control lines could be minimized by causing one or more of these control lines to perform a plurality of functions while retaining the use of only digital signals, a new advantageous engineering tradeoff between complexity and speed of data transfer could be achieved.